1. Field of the Invention
The present invention relates to an image pickup lens with a long focal length, which is suitably used for an image pickup optical system such as a silver-salt film camera, a digital still camera, or a video camera.
2. Description of the Related Art
Previously, an image pickup optical system (a telephoto lens) of so-called a telephoto type that is configured by a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power in order from an object side to an image side has been known as an image pickup optical system having a long focal length.
Japanese Patent Laid-Open NO. H11-109222 discloses an optical system that is configured by using an inexpensive material with a low specific gravity and a diffraction optical element with a large effect of correcting a chromatic aberration without using a material having an abnormal partial dispersibility in order to reduce a weight and a cost of the optical system. Japanese Patent Laid-Open No. 2009-271354 discloses a small-size and light-weight inner focus type telephoto lens in which various kinds of aberrations such as a chromatic aberration have been appropriately corrected by arranging a diffraction optical element in an optical system and also by using an aspherical surface.
However, in Japanese Patent Laid-Open No. H11-109222, it is difficult to reduce the weight since the number of lenses constituting the front lens unit of the telephoto lens is the same as the conventional number. In Japanese Patent Laid-Open No. 2009-271354, a total length of the optical system can be shortened to reduce the weight of a lens barrel, but the sensitivity gets higher. Commonly, the sensitivity for a surface accuracy of the front lens unit arranged at the object side of the telephoto lens and a manufacturing error such as an assembly error is high. There is a tendency that the sensitivity gets higher as the refractive power of the front lens unit is strengthened to shorten the total length compared with the focal length, and that on the other hand the sensitivity gets lower as the refractive power of the front lens unit is weakened to lengthen the total length compared with the focal length.
Particularly, in the conventional art disclosed in Japanese Patent Laid-Open No. 2009-271354, an aspherical surface having a large amount of errors is used as a surface closest to the object side in order to appropriately correct aberrations other than the chromatic aberration, such as a spherical aberration and a field curvature, which are deteriorated by shortening the total length. Accordingly, it is difficult to manufacture the optical system which is disclosed in Japanese Patent Laid-Open No. 2009-271354. Furthermore, in order to reduce the deterioration of the aberration caused by shortening the total length, many lenses are disposed in the front lens unit that effectively corrects the aberration. Therefore, the weight of the lenses may increase even when the weight of the lens barrel decreases, and it is difficult to manufacture the light-weight telephoto lens.